Exhaust gas recirculation (EGR) is used in many internal combustion engines, and particularly gasoline and diesel engines. In an EGR system, a portion of an engine's exhaust gas is recirculated back to the engine cylinders. Therefore, at a time when a cylinder allows fuel, oxygen and other combustion products into the combustion chamber for ignition, vehicle exhaust is also allowed to enter the chamber. The introduction of vehicle exhaust into the combustion chamber has a number of consequences. One consequence is that the introduced exhaust displaces the amount of combustible matter in the chamber. Because the exhaust gases have already combusted, the recirculated gases do not burn again when introduced to the chamber. This results in a chemical slowing and cooling of the combustion process by several hundred degrees Fahrenheit. Thus, combustion of material in the cylinder results in a same pressure being exerted against the cylinder piston as results from combustion without the recycled exhaust, but at a lower temperature. The lower temperature leads to a reduced formation rate for nitrous oxide emissions. Thus, EGR technique results in less pollutants being emitted in an engine's exhaust.
Additionally, the introduction of recirculated exhaust into an engine cylinder allows for an increase in engine performance and fuel economy. As the combustion chamber temperature is reduced, the potential for harmful “engine knock” or engine detonation is also reduced. Engine detonation occurs when the fuel and air mixture in a cylinder ignite prematurely due to high pressure and heat. In engine detonation, instead of an associated spark plug controlling when a cylinder's fuel is ignited, the ignition occurs spontaneously, often causing damage to the cylinder. However, when the combustion chamber temperature is reduced due to EGR, the potential for engine detonation is also reduced. This allows vehicle manufacturers to program more aggressive (and hence, more efficient) timing routines into an associated spark timing program. Because of the aggressive timing routines, the vehicle's power control module (PCM) has a greater advance notice and thus more time to take measures to prevent engine detonation. The aggressive timing routines can also result in higher cylinder pressures leading to increased torque and power output for the vehicle. For these and additional reasons, high levels of EGR are especially useful when applied to turbocharged or supercharged engines.
Thus, EGR systems provide tremendous benefits to motor vehicle engines. However, EGR systems do have limitations. For example, although using an EGR system to recirculate increasingly large amounts of exhaust will result in combustion chambers dropping even further in temperature, high levels of EGR also result in a combustion delay. To a limited extent, this delay can be compensated for by using an appropriate spark timing program. However, if EGR levels are too high, even spark timing programs are not able to correct the situation, and noticeable delays in the generation of power for the vehicle will occur. Additionally, there is generally a delay or transient response time to desired changes in EGR levels, and this transient response time is amplified at high levels of EGR. Accordingly, there is a desire to obtain the benefits of increased EGR without incurring the delay penalties normally associated with high levels of EGR.